1400 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 38, NO. 5, SEPTEMBER/OCTOBER 2002 Design Considerations for a Soft-Switched Modular 2.4-MVA Medium-Voltage Drive Ashish Bendre, Ian Wallace, Glen A. Luckjiff, Steve Norris, Randal W. Gascoigne, Deepak Divan, Fellow, IEEE, and Robert M. Cuzner Abstract—A new six-phase 2.4-MVA soft-switched medium-voltage drive system utilizing series-stacked mod- ules with low-voltage devices has been developed. The drive system combines a new soft-switched dc–dc converter with resonant dc-link inverter technology to deliver extremely low total harmoic distortion sinusoidal output, high power density, and high efficiency. The series-stacked configuration with the associated single-phase loading lead to unique power and control design challenges. Device selection, control of parasitic elements, sensing methods for converter control, custom magnetic compo- nent design, and clamping techniques have lead to a substantial improvement in device voltage utilization. The dc–dc converter controls must regulate the intermediate dc-bus voltage under single-phase loading while balancing transformer excitation and maintaining zero-voltage switching, among other tasks. Proper control of the resonant dc-link inverter requires the selection and tuning of the appropriate modulator and understanding its effect on the power circuit ratings. Index Terms—DC–DC converters, medium-voltage converter, multilevel converter, resonant dc link, sigma–delta modulation, soft switching. I. INTRODUCTION A NEW six-phase 2.4-MVA soft-switched medium-voltage drive system that features extremely low total harmonic distortion (THD), high power density, and high efficiency has been developed in a collaborative effort. The drive is powered from a single 700–900-Vdc source while the phase output is 1380-V line–neutral and 286-A rms. The drive was applied to a high-torque-density permanent-magnet motor and the drive’s main requirement was a stringent output voltage individual harmonic distortion (IHD) of 55 dB in order to minimize the motor torque ripple. This requirement necessitated a high-band- width inverter with a filtered output. As the application also required high power density and high efficiency, the power Paper IPCSD 02–016, presented at the 2001 Industry Applications Society Annual Meeting, Chicago, IL, September 30–October 5, and approved for pub- lication in the IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS by the Indus- trial Power Converter Committee of the IEEE Industry Applications Society. Manuscript submitted for review November 1, 2001 and released for publica- tion June 13, 2002. A. Bendre, G. A. Luckjiff, S. Norris, R. W. Gascoigne, and D. Divan are with Soft Switching Technologies Corporation, Middleton, WI 53562 USA (e-mail: abendre@softswitch.com; gluckjiff@softswitch.com; ddivan@softswitch.com). I. Wallace is with the Eaton Corporation, Milwaukee, WI 53216 USA (e-mail: iantwallace@eaton.com). R. M. Cuzner is with the Advanced Development Group, DRS Power and Control Technology, Eaton Corporation, Milwaukee, WI 53216 USA (e-mail: robertmcuzner@eaton.com). Publisher Item Identifier 10.1109/TIA.2002.802991. conversion needed to be done at high frequencies. High-voltage (2400 V) devices, which have significantly high switching losses, could not be used in the design, as they would violate the efficiency and power density targets. Instead, three inverters using commonly available lower voltage devices at 1200 V, producing 460-V rms at 286-A rms were connected as a series stack to produce the required output voltage. As the input is a single uncontrolled source, a new loss-limited dc–dc converter module was developed to provide isolated, regulated dc voltage to the inverter modules. For the output stage, hard-switched pulsewidth modulation (PWM) inverters with interleaved switching have been shown to achieve low THD [1]. However, the synchronization of control coupled with the higher switching losses makes this approach unattractive. Three-phase resonant dc-link (RDCL) inverter modules that provide high efficiency and power density along with a spread spectrum noiseband allowing independent (asynchronous) operation with extremely low THD have been previously developed [2]. For this work, these modules were converted to single phase and substantially modified to further improve power density and device utilization. Each output phase of the drive system contains three series-connected single-phase RDCL inverters each powered by an isolated dc–dc converter as shown in Fig. 1. The high-power high-frequency series-stacked configuration and the application requirements lead to unique design chal- lenges and tradeoffs for both the inverter and the dc–dc con- verter modules. This topology leads to single-phase loading on the output of the dc–dc converter, utilizes the devices closer to their ratings, and increases voltage stress on isolation bound- aries. These issues affect device selection, magnetic component design, control of parasitic elements, and capacitor and sensor selection for both soft-switched converters; these are some of the major design issues discussed in this paper. The function of the dc–dc converter is to regulate the output dc-bus voltage, while handling single-phase current loading, balancing trans- former excitation, and maintaining zero-voltage switching. This is accomplished by varying the operating frequency from 20 to 30 kHz using state machine control. Control of the RDCL in- verter involved a tradeoff between designing the modulator to produce low THD waveforms and rating the power circuit com- ponents to achieve high power density. II. DC–DC CONVERTER DESIGN The major restrictions to higher frequency high-power dc–dc converters are power device switching loss, throughput loss due 0093-9994/02$17.00 © 2002 IEEE